The stop signal stepping task: how action cancellation commands disrupt step initiation in young and healthy older adults.

Action cancellation - the ability to rapidly cancel an initiated movement in response to unexpected events - has been extensively studied in the upper limb using the stop signal task (SST). During gait, action cancellation is critical to stop and modify steps to avoid unexpected hazards and prevent falls. By adapting the SST to step initiation, this study investigated how the anticipatory postural adjustment (APA) and foot-lift phases of forward stepping were influenced by action-cancellation commands, and whether these processes changed with healthy ageing. Twenty-seven young (M = 28.7 years) and 29 healthy older adults (M = 70.1 years) performed SST variants involving stepping, foot tapping, and finger button-pressing. Across conditions, older adults exhibited slower response execution speed and more slowing when stop cues were anticipated, yet their stopping speed did not differ from that of young adults. When an APA was initiated in a step cancellation trial, the magnitude of the preparatory weight shift toward the step leg did not differ between trials in which foot-lift was successfully cancelled and those in which foot-lift still occurred. Foot-lift could be cancelled when stop cues were presented at similar phases of step preparation for young and older adults. These results suggest that the initial loading of the step leg is ballistic, however as weight transfers toward the stance leg, external stop cues can decouple the APA and foot-lift step phases. While older adults exhibited slower action initiation, a greater reliance on proactive inhibition under conditions of uncertainty may help preserve stopping efficiency.